The present invention relates to a seal and in particular relates to a redundant pressure seal for use between separable sections of a pressure vessel. The redundant pressure seal is usable in a joint between sections of a solid rocket booster which is used with a space shuttle.
The prior art seal of the solid rocket booster includes a cylindrical tang of about 12 feet in diameter on one section, which mates with a cylindrical clevis on the mating section. Two "O" rings are inserted into closely-spaced parallel grooves which are machined into an extension of the inside surface of the clevis. These rings are compressed by the tang section as it is mated with the clevis section. This technique requires maintenance of a stable and uniform gap between the clevis and tang around the complete circumference of the joint in order to maintain proper squeeze on the "O" rings during rapidly changing structural deformations and internal gas pressures.
It should be noted that the entire weight of about 4,500,000 pounds of the space shuttle is borne by the two solid rocket boosters while the space shuttle is at rest on the ground. This results in a huge compressive force on each booster as well as a huge bending moment. The moment occurs because the orbiter is mounted at an offset from the centerline of the external tank. In addition, the combined weight of the orbiter and external tank bears on each booster at a distance from the ground reaction point of the booster. The magnitude of the moment on each booster structure is of the order of 7,000,000 pound-feet while resting on the ground.
The compression of the solid rocket booster structure when combined with the moment due to eccentric loading on the booster when on the ground will result in significant flexing of the booster structure in accordance with principles discussed by S. Timoshenko in his 1936 Engineering Societies Monograph titled "Theory of Elastic Stability". The applicable case is presented in paragraph 4, page 11 of the monograph. This flexing will distort the gap between the tang and clevis and thus adversely impact on the squeeze of the "O" rings, particularly since the structure adjacent to the tank and clevis is the weakest structural element of the booster with regard to resisting bending moments because of looseness resulting from machining tolerances in the shear pins, mating holes, tang and clevis.
The Timoshenko text (page 465) also indicates that thin cylinders tend to become flat when subjected to bending moments. The tang section is more flexible than the clevis section and therefore is more susceptible to becoming oval, introducing a further source of distortion in the "O" ring gap. It is also known that the ends of the sections tend to become oval due to handling during transportation prior to assembly and stacking.
When the boosters are fired for lift-off, there is a large and sudden build up of pressure within the booster which causes the shell to bulgs outward. Here, again, the "O" ring gaps are susceptible to changes because the clevis section is stiffer than the tang section which will bend away from the clevis extension. When the space shuttle is released from the ground, the compressive force in the booster is suddenly changed to a tensile force which causes angular oscillations in the booster structure. This arises because the booster has a high elastic energy stored in its structure when resting on the ground because of the flexure due to the combined compression and moment. This energy is suddenly released at lift-off and causes the booster structures to act like giant vibrating strings. Here, again, the "O" ring gaps are very susceptible to distortion because of looseness at the joints.